A human audio frequency band ranges from 20 Hz to 20 kHz. In case of reproducing sound with a small speaker, it is impossible to obtain a sufficient sound pressure level corresponding to bass whose frequency is equal to or lower than 100 Hz, so that generally the bass is likely to be insufficient. Particularly, in view of such current trend that a display device represented by a liquid crystal television is made thinner, it is an important object to reproduce sound having sufficiently great bass with a small speaker which can be installed in the display device.
FIG. 30 is a graph illustrating an example of frequency characteristic of a small speaker which can be installed in a television. A sound pressure significantly drops at a low frequency equal to or lower than 100 Hz. Actually, a sound pressure at 50 Hz is lower than a sound pressure at 100 Hz by about 20 dB.
As a technique for reproducing sound having sufficient bass with such a small speaker, it is known to adopt a technique in which false feeling of human auditory sense is utilized to enhance the bass in a virtual manner.
For example, an overtone addition device described in Japanese Unexamined Patent Publication No. 95567/1996 (Tokukaihei 8-95567)(Publication date: Apr. 12, 1996) extracts, from an inputted sound signal, a signal corresponding to a predetermined band containing a fundamental wave and mixes overtones of the fundamental wave generated from the extracted signal with the sound signal, so as to output the mixture (see FIG. 1 of the document). According to the overtone addition device, even in case where it is impossible to output a fundamental wave of 100 Hz with a sufficient sound pressure from a speaker, the overtones (200 Hz, 300 Hz, . . . ) can be emphasized so as to output from the speaker the sound processed in this manner. As a result, it is possible to cause a listener to feel as if the fundamental wave of 100 Hz was outputted with a sufficient sound pressure.
Further, a sound enhancement system described in Japanese National Publication of Translated Version No. 524996/2002 (Tokuhyo 2002-524996)(Publication date: Aug. 6, 2002) extracts, from a low frequency signal having been extracted by a low-pass filter, a group of low frequency signals whose bands are different from each other by using a plurality of band-pass filters. Further, the low frequency signal group having been extracted by the band-pass filters are amplified by using a gain-variable amplifier, and then the amplified low frequency signal group is synthesized, thereby obtaining a low frequency signal to be mixed with an inputted sound signal (see FIG. 16 of the document).
However, in each of the foregoing conventional devices, an output level of the output signal exceeds an acceptable level (clip level) of a receiving end device (a D/A converter, a power amplifier, and the like), which results in such a problem that sound finally outputted from the speaker is distorted.
This problem is more specifically described as follows.
In the overtone addition device described in Tokukaihei 8-95567, the inputted sound signal and the generated overtones are respectively amplified by an amplifier so as to be mixed with each other. Thus, even if an input level (amplitude of the sound signal) does not cause any clipping in a receiving end device, an output level often exceeds the acceptable level of the receiving end device due to amplification of the sound signal or addition of the amplified overtones to the sound signal. As a result, sound outputted from the speaker is likely to be distorted.
Further, in the sound enhancement system described in Tokuhyo 2002-524996, a gain of the amplifier for amplifying the low frequency signal group extracted with the plurality of band-pass filters is controlled in accordance with low frequency signals to be inputted to the band-pass filters. Thus, clipping less occurs than the overtone addition device described in Tokukai 8-95567. However, the gain of the amplifier of the sound enhancement system is controlled in accordance with the low frequency signals which have not been inputted to the band-pass filters, so that the acceptable level of the receiving end device is easily exceeded by an output level of an output signal outputted after (i) a stage in which the low frequency signal group having passed through the band-pass filters is amplified, (ii) a stage in which the amplified low frequency signal group is synthesized, and (iii) a stage in which the synthesized low frequency signals are mixed with the original sound signal. Further, if the gain of the amplifier is set to be low so as not to cause the clipping, the low frequency signal group is not sufficiently amplified, which results in such a problem that sufficient bass cannot be obtained.